Susceptors are often used in conventional microwave heating packages to enhance the heating, browning, and/or crisping of food items. A susceptor generally comprises a thin layer of microwave energy interactive material (generally less than about 100 angstroms in thickness, for example, from about 60 to about 100 angstroms in thickness, and having an optical density of from about 0.15 to about 0.35, for example, about 0.17 to about 0.28) that tends to absorb at least a portion of impinging microwave energy and convert it to thermal energy (i.e., heat) at the interface with the food item. Susceptors are typically supported on a microwave energy transparent substrate, for example, a polymer film, thereby collectively forming a “susceptor film”. Susceptor films, in turn, are often joined (e.g., adhered) to a dimensionally stable supporting material (or “support”), for example, paper, paperboard, or a polymer film, to collectively define a “supported susceptor film”.
Supported susceptor films may be used alone or in combination with numerous other materials to form various microwave heating packages, cartons, or other constructs. In many cases, a “patch” (i.e., a piece) of supported susceptor film is applied to a microwave heating package in one or more areas to provide the desired level of heating, browning, and/or crisping of the food item.
In many instances, the package or carton may generally be erected from a flat blank comprising a disposable material, for example, a paper-based material such as paper or paperboard. Such paper-based materials generally exhibit alignment of fibers in the machine direction (MD), such that the length of the fiber extends along the machine direction and the width of the fiber extends along the cross direction (CD) (or cross machine direction) of the paper-based material (e.g., paper or paperboard).
It has been observed that in many freezers (e.g., grocer's freezers), where the microwave heating package may be subjected to periodic thaw cycles (in which warm air is introduced into the freezer to prevent frost buildup), the panel or portion of the package to which the supported susceptor is joined may tend to buckle or warp, typically in the unadhered (e.g., unglued) areas. While not wishing to be bound by theory, it is believed that this warping or buckling is due to the change in humidity of the freezer during the thaw cycles. As the humidity increases, the fibers tend to absorb water and expand. The fibers tend to expand to a greater extent in a direction perpendicular to the orientation of the fibers, i.e., through the width of the fibers, rather than the length. As a result, the paper or paperboard tends to buckle or warp in the cross direction (CD) of the panel. It has also been observed that the degree and pattern of buckling may depend on the pattern of adhesion of the susceptor patch.
It has been observed that using a full coverage adhesive may address this problem. However, such structures have been shown to be prone to delamination during heating. While not wishing to be bound by theory, it is believed that during heating, the moisture in the support layer and/or adhesive is released as water vapor, which exerts a pressure on the adjacent layers of the structure. With insufficient pathways for the water vapor to escape, the layers of the structure tend to delaminate and loft away from one another. In some cases, this lofting or pillowing of the structure can cause a food item seated on the structure to be turned over or toppled undesirably.
It has been suggested that using of a patterned adhesive may alleviate this problem. For example, International Patent Application No. PCT/US09/063963, filed Nov. 11, 2009, which is incorporated by reference herein in its entirety, discloses the use of a patterned adhesive in various susceptor structures. The spaces between the adhered areas are believed to serve as pathways for transporting water vapor away from the structure, thereby preventing delamination of the adjoined layers.
Accordingly, there is a need for a package including a supported susceptor film that can withstand the absorption of moisture during a thaw cycle in a freezer without warping. There may further be a need in some instances for a package including a susceptor film that can further allow for the release of moisture from the support layer during microwave heating to prevent delamination.